Halogenoalkanes,
also known as haloalkanes or alkyl halides, are organic compounds in which one
or more hydrogen atoms in an alkane have been replaced by halogen atoms,
fluorine, chlorine, bromine or iodine. In carbon-halogen bond, halogens have
significantly greater electronegativities than carbon except iodine. In result,
this functional group is polarized so that the carbon is electrophilic and the
halogen is nucleophilic. Halogenoalkanes are can be classified depending on the
halogen atom position on the chain of carbon atoms. The carbon which is attached
with the halogen atom is linked up with only one other alkyl group in primary
halogenoalkanes, whereas directly linked up with two and three other alkyl
groups in secondary halogenoalkanes and tertiary halogenoalkanes respectively.
In some case, primary halogenoalkanes are counted even though there are no alkyl
groups attached to the carbon with the halogen on it. Three characteristics
provide important influences on the chemical behavior of halogenoalkanes, these
are electronegativity, covalent bond strength and the relative stability of the
corresponding halide anions. Fluoroalkanes have the strongest of the
carbon-halogen covalent bonds so that they are unreactive. This is stronger
single bond than a carbon-carbon bond. The carbon-chlorine covalent bond is
slightly weaker than a carbon-carbon bond, and the bonds to the other halogens
are weaker. The stability may be estimated from the relative acidities of the
H-X acids. All the hydrohalic acids are very strong, but with small differences
in the direction HCl < HBr < HI, with the exception of HF. Halogenoarenes,
also called haloarene, or aryl Halide, are an organic compound in which one or
more hydrogen atoms in an aromatic ring have been replaced by halogen atoms. The
Haloarenes exhibit many differences compare to haloalkanes in the method of
preparation and their chemical and phisical properties. Haloalkanes are used in
as refrigerants, solvents, blowing agents, aerosol propellants, fire
extinguishing media , and in semiconductor device fabrication. One of big
consumption of halogenoalkanes (properly speaking, halogenoalkenes) is as a raw
material to prepare plastics such as PVC [poly(chloroethene)] from chloroethene
and PTFE [poly(tetrafluoroethene)] from tetrafluoroethene. Halogenoalkanes and
halogenoarenes react with lots of compounds resulting in a wide range of
different target substances. They are useful intermediates in making other
organic compounds.

Summary of
substitution and elimination reaction based on alkyl group
structure

Alkyl
Halides

SN1 and
E1

SN2 and
E2

Primary (RCH2X)

Neither SN1 nor E1
occur

both
SN2 and E2 will
occur(SN2 preferred. E2
preferred when sterically hindered base is used)